It's the amount of space you need to leave between your car and a blaring, flashing emergency vehicle.

It's a football field and a half.

It's 152.4 meters of inflated yellow-and-blue vinyl your entire back side can glide down on Saturday and Sunday, thanks to the Lyon Street Waterslide, presented by nonprofit group Rob Bliss Events.

At what speed will you launch down the downtown hill? Nobody knows. At least, not yet.

Test rides on the slide will begin after Lyon Street is shut down at 6 p.m. Friday and professional crews inflate and anchor the form-fitted slide to the asphalt. (Lyon Street will reopen in time for Monday morning traffic, said Todd Tofferi, the city's special events coordinator.)

If testing proves the slide is safe for potential riders, the slide will open at 8 a.m. Saturday. Events coordinator Rob Bliss, 21, will take the first plunge down the slide.

Then, it's your turn.

But there are a couple rules to follow. Here's a deluge of information that will come in handy if you want to head downtown to check out the slide and perhaps be daring enough to ride it.

1. Do not grease yourself like a stuck pig -- You will be turned away. "We don't need someone going 100 miles-an-hour down Lyon, in a car or on their own," Bliss said.

2. It's a waterslide, so wear a bathing suit -- T-shirts can be worn, but the less you're wearing, the better. Keep jewelry and accessories to a minimum, or, even better, at home. A deflated waterslide thanks to torn vinyl is no fun for anyone. If you're wearing something questionable or unreasonable, expect to be turned away.

3. It's not you, it's just genetics -- Riders have to be 48 inches or taller. Bliss and his team of professionals will determine after testing the slide Friday whether children or individuals who are smaller than 48 inches will be able to ride down the slide with someone else. So stay tuned.

4. No fancy stuff -- Riders can only slide down on their backs. You cannot sit up, since netting encases almost the entire length of the slide ensuring no one can be ejected. There are a few spaces where the net breaks to allow for anyone to get out if they get stuck.

5. What? You're hungry and you need to go to the bathroom? -- Food vendors will be set up for hungry children. And 14 porta-jons will be available at the far end of Grand Rapids Community College's outdoor courtyard on Bostwick Avenue near Fountain Street.

6. How are lines going to work? -- Both sidewalks on either side of Lyon Street will be used for riders waiting in line. The sidewalk on the right side of the street will be for the right lane of the waterslide, and vice versa.

7. I don't "do" lines. -- No problem. No patience for lines means there are $30 skip-the-line tickets, available at booths stationed at the beginning of the slide at the top of the hill and also at the bottom. T-shirts emblazoned with 'I rode the Lyon Street Waterslide," also will be available for $10 at the booths. Shirts were printed by Grand Rapids Graphics.

8. What if there is a medical emergency? -- EMTs will be on hand, as well as DK Security officers, Grand Rapids Police and GRCC security.

IF YOU GO

9. When will be the best time to ride? -- Sunday morning. Sunday, in general, will have fewer people, Bliss believes. Of course, if you want to feel the climax of crowd excitement, come see the first few riders descend the beast Saturday morning at 8 a.m.

10. Where do I park? -- GRCC is opening its usually student-occupied ramp on Bostwick to the public for $3 per car. The ramp holds about 1,700 vehicles. Tofferi recommends the use of several downtown parking ramps that will be open for waterslide traffic. Prices will vary.

11. Isn't a lot of water going to be used? -- Actually, not really. Bliss said his crews estimate they'll use 500 gallons per hour. By the end of day two, Tofferi said about 15,000 gallons in total might be used, which is the size of a regular underground backyard pool.

12. What about next year? -- Bliss hopes the waterslide becomes an annual event for the next 20 years -- the projected lifespan of the slide, which will be stored at GRCC. Bliss said he hopes to double the length of the slide, making it 1,000 feet, for next year.